The present invention involves the generation and detection of very high frequency surface acoustic waves in a substrate. The structure of a substrate affects the frequency of the surface acoustic waves. Understanding the nominal structure of a substrate and modeling how variations in the nominal structure would affect the propagation of surface acoustic waves provides a key to decoding structure properties by observing surface acoustic wave propagation.
Surface acoustic waves such as Rayleigh waves and/or Lam waves are induced in a substrate such as stack of films on a semiconductor material or “film stack” using a typical pump/probe beam arrangement of the type embodied by Rudolph's MetaPULSE metrology system. A difference between a standard opto-acoustic system (as exemplified by U.S. Pat. No. 6,400,449) or a contact mask opto-acoustic system (as exemplified by U.S. Pat. No. 6,381,019) and the present invention is that a noncontact mask or pattern of light is projected onto the surface of the substrate, the pattern of light inducing a corresponding stress concentration pattern in the surface of the substrate. As will be appreciated, the stress concentrations propagate as acoustic waves both into the body of the substrate (longitudinal waves) as well as along the surface of the substrate. While it is possible to measure the propagation of acoustic waves “down” into the surface of a substrate, it is also possible to measure surface acoustic waves. It is also possible to measure the effects of surface and “depth” acoustic waves separately or together and to use this data to appreciate more and different features and characteristics of the substrate that is under test.